Quantizing apparatus and quantizing method

ABSTRACT

A subtraction device subtracts a predicted value generated by a predicting portion from a digital picture signal supplied through an input terminal. The subtraction device generates a difference signal as an output signal. A quantizing portion detects an activity of the difference signal, designates the number of quantizing steps corresponding to the activity, and quantizes the difference signal with the number of quantizing steps. The quantizing portion outputs side information to an output terminal.

This application is a continuation of Ser. No. 08/576,315, filed Dec.21, 1995, now U.S. Pat. No. 5,870,434.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a quantizing apparatus and method forquantizing the difference between an input signal (such as a digitalaudio signal or a digital picture signal) and a predicted valuegenerated therefrom.

2. Description of the Related Art

A prediction encoding method for compressing the transmissioninformation amount of a digital audio signal, a digital picture signal,and so forth is known. For example, in one-dimensional DPCM, thedifference between an input sample value and a predicted value is formedin time direction. On the other hand, in two-dimensional DPCM, thedifference between an input sample value and a predicted value is formedin spatial direction. Since a digital information signal has acorrelation in time direction and in spatial direction, the differenceconcentrates on around 0. Thus, the difference signal can be quantizedwith the number of bits smaller than the number of quantizing bits.Consequently, the information amount can be reduced. In addition, whenthe variable length encoding process is performed using thecharacteristic of the concentration of the distribution of thedifference signal, the information amount can be more reduced.

In the distribution of the frequency of the difference signal, valuesconcentrate on around 0. Thus, in a conventional quantizing apparatusthat deals with the difference signal, the quantizing step width ataround 0 is finely designated. As the level becomes large, thequantizing step width is coarsely designated. This quantizing apparatusis referred to as a non-linear quantizing apparatus. In the conventionalquantizing apparatus including the non-linear quantizing apparatus, allpossible levels of the difference signal are quantized. For example,when one sample (one pixel) of a digital picture signal is quantizedwith eight bits, the values of the difference signal range from -255 to+255. In the conventional quantizing apparatus, all the range is usedfor the quantizing process.

In the conventional quantizing apparatus, the number of quantizing stepsis restricted to even numbers so as to represent quantized values inbinary notation. In the case that the number of quantizing steps isrestricted to even numbers, when difference data is quantized with 0 asa decoded value, the quantized values are not symmetrical with respectto 0. Alternatively, even if the number of quantizing steps is 2n-1,since 2n should be used as the number of quantizing steps, there is aloss.

When the number of quantizing bits is changed from 2 to 3, the number ofquantizing steps is increased from 4 to 8. Thus, as this example shows,the number of quantizing steps largely varies corresponding to thenumber of quantizing bits. This means that the variation of the numberof quantizing bits for keeping the transmission data amount constantcauses the quantizing step width to largely vary, thereby largelychanging the restored picture quality.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a quantizing apparatusand method that is regardless of whether the number of quantizing stepsis an odd number or an even number and that suppresses the variation ofthe number of quantizing steps.

According to an aspect of the present invention, there is provided aquantizing apparatus, comprising, a device for receiving an inputdigital signal and quantizing circuit for quantizing the input digitalsignal corresponding to the number of quantizing steps including an oddstep number so as to determine a quantized code and for matching avariable-length code with the quantized code so as to generate aquantized value.

The present invention further includes a memory for receiving thequantized code and having a code converting table for outputting avariable-length quantized value corresponding to the quantized code,wherein the quantized value is transmitted.

According to another aspect of the invention, there is provided aquantizing method, comprising the steps of, receiving an input digitalsignal, and quantizing the input digital signal corresponding to thenumber of quantizing steps including an odd step number so as todetermine a quantized code and matching a variable-length code with thequantized code so as to generate a quantized value.

When the difference data (difference signal) is quantized, even if 0 isincluded in a decoded value, the difference data can be symmetricallyquantized with respect to 0. When the optimum number of quantizing stepsis 2n-1, the difference data can be quantized with the optimum number ofquantizing steps.

The above, and other, objects, features and advantage of the presentinvention will become readily apparent from the following detaileddescription thereof which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams showing a prediction code encoderaccording to the present invention;

FIG. 2 is a block diagram showing a prediction code decoder according tothe present invention;

FIG. 3 is a schematic diagram for explaining an example of predictionencoding process;

FIG. 4 is a block diagram showing a quantizing apparatus according to anembodiment of the present invention;

FIG. 5 is a schematic diagram for explaining the number of quantizingsteps;

FIG. 6 is a schematic diagram for explaining a variable length encodingprocess according to the present invention;

FIG. 7 is a schematic diagram for explaining an example of ahierarchical encoding process according to the present invention;

FIG. 8 is a schematic diagram for explaining an example of a thehierarchical encoding process;

FIG. 9 is a block diagram showing an example of the construction of anencode side of the hierarchical encoding process; and

FIG. 10 is a block diagram showing an example of the construction of adecode side of the hierarchical encoding process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, with reference to the accompanying drawings, an embodiment of thepresent invention will be described. FIGS. 1A and 1B show examples of aprediction code encoder that generates a difference signal. In FIG. 1A,for example a digital picture signal is supplied to an input terminal11. The digital picture signal is supplied to a predicting portion 12and a subtraction device 13. The subtraction device 13 subtracts apredicted value generated by the predicting portion 12 from each pixelvalue and generates a difference signal as an output signal.

The difference signal is supplied to a quantizing portion 15 through ablock segmenting circuit 14. The quantizing portion 15 quantizes thedifference signal with the number of quantizing bits smaller than theoriginal number of quantizing bits. The present invention is applied forthe quantizing portion 15 and an embodiment thereof is shown in FIG. 4.The quantizing portion 15 generates a quantized output (quantized value)and side information. The quantized value is obtained from an outputterminal 16. The side information is obtained from an output terminal17. In reality, as shown in FIG. 1B, the output signal of thesubtraction device 13 is supplied to a local decoding device 12'. Adecoded output signal is supplied from the local decoding device 12' tothe subtraction device 13.

FIG. 2 shows a decoder corresponding to the encoder shown in FIGS. 1Aand 1B. A quantized value and side information are supplied to inputterminals 21 and 22, respectively. The quantized value and the sideinformation are supplied to a dequantizing portion 24. The dequantizingportion 24 dequantizes the quantized value to a dequantized value(representative value).

The dequantized value is supplied from the dequantizing portion 24 to anaddition device 25. An output signal of the addition device 25 issupplied to an output terminal 27 and a predicting portion 26. Thepredicting portion 26 generates a predicted value and supplies it to theaddition device 25. When a sample value for refreshing is periodicallyinserted for preventing errors of the encoder from being cumulated, theaddition device 25 supplies the dequantized value to the output terminal27 without performing the addition.

FIG. 3 shows a part of one screen for explaining an example of theprediction. In FIG. 3, a to h represent locally decoded pixel values. Ato P represent pixel values that have not been encoded. A predictedvalue A' corresponding to the pixel value A is formed of adjacentlocally-decoded pixel values. A predicated value of the pixel value A isformed of for example A'=4c-3(b-f), A'=f+c-d, and so forth. A predictedvalue of the pixel value B is formed of a locally decoded value by thesimilar arithmetic operations.

For example, a predicted value (for example, A') is subtracted from areal pixel value (for example, A). (Namely, Δa=A-A' is calculated).Thus, the difference Δa is formed. As shown in FIG. 3, the difference isblock segmented into a block composed of 4 pixels×4 pixels by the blocksegmenting circuit 14. When a digital audio signal is handled, apredicate value is formed in time direction and thereby a block of aone-dimensional difference signal is formed.

FIG. 4 shows a detailed construction of the quantizing portion 15according to the embodiment. A difference signal supplied from thesubtraction device 13 is supplied to the quantizing portion 15. Thequantizing portion 15 includes a ROM 32, a quantizing device 34 and anactivity detecting circuit 33. As described above, the block segmentingcircuit 32 segments the difference signal into a block composed of 4pixels×4 pixels. The block-segmented data is supplied to the activitydetecting circuit 33 and the quantizing device 34.

As an example of an activity determining method performed by theactivity detecting circuit 33, a dynamic range is used. Besides thedynamic range, it is possible to use the sum of the absolute values ofthe differences against the average value, the absolute values of thestandard deviation, and so forth as activities.

The activity detecting circuit 33 determines a quantizing characteristiccorresponding to the detected activity. In this example, the activitydetecting circuit 33 determines the number of quantizing steps andsupplies a control signal that represents the number of quantizing stepsto the quantizing device 34 and the ROM 32. The quantizing device 34quantizes the data of the block-segmented difference signal with thenumber of the quantizing steps. An ID that represents the number ofquantizing steps as side information is supplied from the activitydetecting circuit 33 to the output terminal 17. A quantized code and theID are supplied to the ROM 32 as its address. A variable length code isread out from the ROM 32, and the code is supplied to an output terminal35.

Since the quantizing portion 15 is constituted so as to integrate thequantizing device 34 and the ROM 32 for variable length coding, unlikewith a conventional even number of quantizing steps, an odd number ofquantizing steps can be used. In other words, the quantized value of thevariable length code process is transmitted or recorded. Thus, beforethis process, the number of quantizing steps can be freely selected.Consequently, when the variable length code converting table stored inthe ROM 32 is properly designed, the information amount to betransmitted can be suppressed from increasing. Next, an example of thecode converting table will be described. In this example, the number ofquantizing steps is 3 for simple explanation. In FIG. 5, the relationbetween input values (quantized codes from the quantizing device 34)(-255 to +255) and output values read out from the ROM 32 (-255 to +255)are shown. When the number of quantizing steps is an odd number (3), therepresentative value of one quantizing code is always 0. In addition, aquantizing step width Δ can be symmetrically designated with respect tozero.

The activity detecting circuit 33 detects the maximum values MAX and theminimum value MIN of the level distribution of the difference signal ofthe block and calculates the dynamic range DR (=MAX-MIN) in theabove-descried manner. When the detected dynamic range DR is in therange from 0 to 63, the number of quantizing steps is designated to 1.When the detected dynamic range DR is in the range from 64 to 128, thenumber of quantizing steps is designated to 3. When the detected dynamicrange DR is in the range from 129 to 191, the number of quantizing stepsis designated to 5. When the detected dynamic range DR is in the rangefrom 192 to 255, the number of quantizing steps is designated to 7. Thequantizing step width Δ is designated corresponding to the odd number ofquantizing steps.

The table shown in FIG. 6 is stored in the ROM 32. This table definesthe relation among the number of quantizing steps, the quantizing code,and the Huffman code as an example of variable length code. As shown inFIG. 6, when the number of quantizing steps is 1, the quantizing code is1 and the Huffman code is 0. When the number of quantizing steps is 3,the quantizing code is 1, 2, or 3. When the quantizing code is 1, theHuffman code is 10. When the quantizing code is 2, the Huffman code is0. When the quantizing code is 3, the Huffman cod is 11. When the numberof quantizing steps is 5, the quantizing code is 1, 2, 3, 4, or 5. Whenthe quantizing code is 1, the Huffman code is 1100. When the quantizingcode is 2, the Huffman code is 100. When the quantizing code is 3, theHuffman code is 0. When the quantizing code is 4, the Huffman code is101. When the quantizing code is 5, the Huffman code is 1101.

When the number of quantizing steps is 7, the quantizing code is 1, 2,3, 4, 5, 6, or 7. When the quantizing code is 1, the Huffman code is00100. When the quantizing code is 2, the Huffman code is 000. When thequantizing code is 3, the Huffman code is 01. When the quantizing codeis 4, the Huffman code is 10. When the quantizing code is 5, the Huffmancode is 11. When the quantizing code is 6, the Huffman code 0011. Whenthe quantizing code is 7, the Huffman code is 00101. Thus, a smallnumber of bits are designated to a quantizing code corresponding tolevel 0 that has a large occurrence frequency. In contrast, a largenumber of bits are designated to a quantizing code that has a smalloccurrence frequency. Consequently, the number of bits to be transmittedcan be totally reduced. The selection of the number of quantizing stepsand the variable length code table are only examples. Thus, it should benoted that another quantizing characteristic and another variable lengthcode can be used.

The present invention can be applied for a quantizing apparatus for ahierarchical code encoder that will be descried later as well as theprediction code encoder shown in FIG. 1. In the following hierarchicalencoding apparatus, predications are performed between hierarchicallevels. By using simple arithmetic expressions for the hierarchicaldata, the number of pixels to be encoded can be prevented fromincreasing.

Next, with reference to FIG. 7, the hierarchical encoding method will bedescribed. FIG. 7 is a schematic diagram showing that the firsthierarchical level is the lowest hierarchical level (original picture)and the fourth hierarchical level is the highest hierarchical level. Forexample, as a higher hierarchical level data generating method, when anaveraging method for averaging four spatially corresponding pixels in alower hierarchical level is used, assuming that the higher hierarchicaldata is denoted by M and the lower hierarchical pixel values are denotedby x₀, X₁, x₂, and X₃, the number of pixels to be transmitted is still4, not increases.

In other words, using M, X₀, X₁, and X₂, the not-transmitted pixel x₃can be easily restored by the following simple arithmetic expression.

    X.sub.3 =4·M-(X.sub.0 +X.sub.1 +X.sub.2)          (1)

Each hierarchical data is generated by averaging the four pixels in thelower hierarchical level. Thus, even if data of the hatched portions inthe drawing, all data can be restored corresponding to the formula (1).

FIG. 8 shows an example of the structure of five hierarchical levels ofhierarchical data formed by the averaging method. It is assumed that thefirst hierarchical level is the level with the resolution of the inputpicture. In the first hierarchical level , data is composed in a blocksize (1×1). In the second hierarchical level, data is composed byaveraging four pixels in the first hierarchical level. In this example,data X₂ (0) in the second hierarchical level is generated by the averagevalue of data X₁ (0) to X₁ (3) in the first hierarchical level. Data X₂(1) to X₂ (3) adjacent to X₂ (0) in the second hierarchical level aregenerated by averaging respective four pixels in the first hierarchicallevel. In the second hierarchical level, data is composed in a blocksize (1/2×1/2).

Data in the third hierarchical level is generated by averaging spatiallycorresponding four pixels in the second hierarchical level. Likewise,data the third hierarchical level is composed in block size (1/4×1/4).Likewise, data in the fourth hierarchical level is controlledcorresponding to data in the third hierarchical level. Data in the thirdhierarchical level is composed in a block size (1/8×1/8). Data X₅ (0) inthe fifth hierarchical level, which is the highest hierarchical level,is generated by averaging data X₄ (0) to X₄ (3) in the fourthhierarchical level. Data in the fifth hierarchical is composed in ablock size (1/16×1/16).

By applying a class categorizing adaptive predicting process for data ina higher hierarchical level, data in a lower hierarchical level can bepredicted. By generating the difference between the data in the lowerhierarchical level and the predicted value (namely, the differencesignal), the signal power can be reduced. Next, an example of theconstruction for reducing the signal power will be described withreference to a block diagram shown in FIG. 9. FIG. 9 shows an example ofthe construction of the hierarchical code encoder. Data d0 in the firsthierarchical level is supplied as input picture data d0 to an averagingcircuit 72 and a subtraction device 76 through an input terminal 71. Thedata in the first hierarchical level is picture data with the originalresolution.

The averaging circuit 72 performs a 1/4 averaging process of a (2pixels×2 pixels) block shown in FIG. 8 for the input pixel data d0 andgenerates hierarchical data d1. The hierarchical data d1 accords withdata in the second hierarchical level shown in FIG. 8. The hierarchicaldata d1 is supplied to an averaging circuit 73 and a subtraction device77.

The averaging circuit 73 performs the same process as the averagingcircuit 72 for the hierarchical data d1. The hierarchical data d2accords with data in the third hierarchical level. The generatedhierarchical data d2 is supplied to an averaging circuit 74 and asubtraction device 78. The averaging circuit 74 performs the sameprocess as the averaging circuits 72 and 73 for the hierarchical data d2and generates the hierarchical data d3. The hierarchical data d3 accordswith data in the fourth hierarchical level. The generated hierarchicaldata d3 is supplied to an averaging circuit 75 and a subtraction device79. The averaging circuit 75 performs the same process as the averagingcircuits 72, 73, and 74 for the hierarchical data d3 and generates thehierarchical data d4. The hierarchical data d4 accords with data in thefifth hierarchical level. The generated hierarchical data d4 is suppliedto a quantizing device 84.

Data in five hierarchical levels is predicted between each hierarchicallevel. In the fifth hierarchical level, the quantizing portion 84performs the quantizing process for compressing data. Output data d21 ofthe quantizing portion 84 is supplied a dequantizing portion 88. Outputdata of the quantizing portion 84 is obtained as data in the fifthhierarchical level to an output terminal 106. Output data d16 of thedequantizing portion 88 is supplied to a class categorizing adaptivepredicting circuit 92.

The class categorizing adaptive predicting circuit 92 performs thepredicting process for the data d16 and generates a predicted value d12of the data in the fourth hierarchical level. The predicted value d12 issupplied to a subtraction device 79. The subtraction device 79 obtainsthe difference between the hierarchical data d3 supplied from theaveraging circuit 74 and the predicted value d12 and supplies adifference value d8 to a quantizing portion 83.

The quantizing portion 83 performs the same compressing process as thequantizing portion 84. Output data of the quantizing poriton 83 issupplied to an arithmetic operation device 96 and a dequantizing portion87. The arithmetic operation device 96 decimates one pixel from fourpixels. Data d20 from the arithmetic operation device 96 is obtained asdata in the fourth hierarchical level from an output terminal 105.

Output data d15 of the dequantizing portion 87 is supplied to a classcategorizing adaptive predicting circuit 91. The class categorizingadaptive predicting circuit 91 performs the predicting process for thedata d15 and generates a predicted value d11 of the data in the thirdhierarchical level. The predicted data d11 is supplied to thesubtraction device 78. The subtraction device 78 obtains the differencebetween the data d2 supplied from the averaging circuit 3 and thepredicted value d11 and supplies a difference value d7 to a quantizingportion 82.

Output data of the quantizing portion 82 is supplied to an arithmeticoperation device 95 and a dequantizing portion 86. The arithmeticoperation device 95 decimates one pixel from fourth pixels. Data d19 inthe third hierarchical level from the arithmetic operation device 95 isobtained as data in the third hierarchical level from an output terminal104.

Output data d14 of the dequantizing portion 86 is supplied to a classcategorizing adaptive predicting circuit 90. The class categorizingadaptive predicting circuit 90 performs the predicting process for thedata d14 and generates a predicted value d10 of data in the secondhierarchical level. The predicted value d10 is supplied to thesubtraction device 77. The subtraction device 77 obtains the differencebetween the data d1 supplied from the averaging circuit 72 and thepredicted value d10 and supplies a difference value d6 to a quantizingportion 81.

Output data of the quantizing poriton 81 is supplied to an arithmeticoperation device 94 and a dequantizing portion 85. The arithmeticoperation device 94 decimates one pixel from four pixels. Data d18 inthe second hierarchical level from the arithmetic operation device 94 isobtained as data in the second hierarchical level from an outputterminal 103.

Output data d13 of the dequantizing portion 85 is supplied to a classcategorizing adaptive predicting circuit 89. The class categorizingadaptive predicting circuit 89 performs the predicting process for thedata d13 and generates a predicted value d9 of data in the firsthierarchical level. The predicted value d9 is supplied to thesubtraction device 76. The subtraction device 76 obtains the differencebetween the input pixel data d0 supplied from the input terminal 71 andthe predicted value d9 and supplies a difference value d5 to aquantizing portion 80.

Output data of the quantizing portion 80 is supplied to an arithmeticoperation device 93. The arithmetic operation device 93 decimates onepixel from four pixels. Data d17 in the first hierarchical level fromthe arithmetic operation device 93 is obtained as data in the firsthierarchical level from an output terminal 102.

The quantizing portions 80 to 84 have same constitutions as shown inFIG. 4, respectively. That is, the quantizing portion can quantize thedata of the block-segmented difference by the odd number of quantizingsteps defined in response to the detected activity, and has the ROM forvariable length coding. The dequantizing portions convert the equantizedvalue having variable length to the quantized code, and then convert thequantized code to the representative value, respectively.

The class categorizing adaptive predicting circuits 89, 90, 91, and 92categorizes classes of pixels in lower hierarchical levels to bepredicted corresponding to the level distribution of a plurality ofspatially adjacent pixels (included in the higher hierarchical levels).A table for predicted coefficients corresponding to individual classesor predicted values that have been learnt is stored in memory. Aplurality of predicted coefficients for individual classes or onepredicted value is read from the memory. The predicted value is used asit is. Predicted coefficients and a plurality of pixels are linearlycombined so as to generate a predicted value. Such a class categorizingadaptive predicting method has been disclosed in Japanese PatentApplication No. HEI 4-155719 by the applicant of the present invention.

FIG. 10 shows an example of the construction of a hierarchical codedecoder corresponding to the encoder. Data in individual hierarchicallevels generated by the encoder is input as d30 to d34 to terminals 131,132, 133, 134 and 135. The data of each hierarchical level is suppliedto dequantizing portion 146, 147, 148, 149, and 150, respectively.

The dequantizing portion 150 performs the decoding process for the inputdata d34 in the fifth hierarchical level and generates picture data d39.The picture data d39 is supplied to a class categorizing adaptivepredicting circuit 162 and an arithmetic operation device 158. Inaddition, the picture data d39 is obtained as picture output data in thefifth hierarchical level from an output terminal 167.

The class categorizing adaptive predicting circuit 162 performs theclass categorizing adaptive predicting process for picture data in thefourth hierarchical level and generates a predicted value d47 of data inthe fourth hierarchical level. An addition device 154 adds data d38(namely, a difference value) supplied from the dequantizing portion 149and the predicted value d47 and supplies picture data d43 to thearithmetic operation device 158. The arithmetic operation device 158performs the arithmetic operation of the formula (1). Thus, all pixelvalues in the fourth hierarchical level are restored from the picturedata d39 supplied from the dequantizing portion 150 and from the picturedata d43. All the pixel values restored by the arithmetic operationdevice 158 are supplied as picture data d51 to a class categorizingadaptive predicting circuit 161 and an arithmetic operation device 157.In addition, the picture data d51 is output as output data in the fourthhierarchical level from an output terminal 166.

The class categorizing adaptive predicting circuit 161 performs theclass categorizing adaptive predicting process for the picture data inthe third hierarchical level in the same manner as described above andgenerates a predicted value d46 in the third hierarchical level. Anaddition device 153 adds data d37 supplied from the dequantizing portion148 and the predicted value d46. Picture data d42 of the addition device153 is supplied to the arithmetic operation device 157. The arithmeticoperation device 157 performs the arithmetic operation of the formula(1). Thus, all pixel values in the third hierarchical level are restoredfrom the picture data d51 supplied from the arithmetic operation device158 and from the picture data d42. All the restored pixel values aresupplied as picture data d50 to a class categorizing adaptive predictingcircuit 160 and an arithmetic operation device 156. In addition, thepicture data d50 is obtained as output data in the third hierarchicallevel from an output terminal 165.

The class categorizing adaptive predicting circuit 160 performs theclass categorizing adaptive predicting process for picture data in thesecond hierarchical level in the same manner as described above andgenerates a predicted value d45 of data in the second hierarchicallevel. An addition device 152 adds data d36 supplied from thedequantizing portion 147 and the predicted value d45. Picture data d41is output from the addition device 152 and supplied to the arithmeticoperation device 156. The arithmetic operation device 156 performs thearithmetic operation of the formula (1). Thus, all pixel values in thesecond hierarchical level are restored from the picture data d50supplied from the arithmetic operation device 157 and from the picturedata d41. All the restored pixel values are supplied as picture data d49to a class categorizing adaptive predicting circuit 159 and anarithmetic operation device 155. In addition, the picture data d49 isobtained as output data in the second hierarchical level from an outputterminal 164.

The class categorizing adaptive predicting circuit 159 performs theclass categorizing adaptive predicting process for picture data in thefirst hierarchical level in the same manner as described above andgenerates a predicted value d44 of data in the first hierarchical level.An addition device 151 adds data d35 supplied from the dequantizingportion 146 and the predicted value d44. Picture data d40 is output fromthe addition device 151 and supplied to an arithmetic operation device155. The arithmetic operation device 155 performs the arithmeticoperation of the formula (1). Thus, all pixel values in the firsthierarchical level are restored from the picture data d49 supplied fromthe arithmetic operation device 156 and the picture data d40. All therestored pixel values are supplied as picture data d48 from thearithmetic operation device 155 and obtained as output data in the firsthierarchical level from an output terminal 163. In the hierarchicalencoding method for preventing the number of pixels to be encoded fromincreasing, the encoding efficiency can be improved.

As a real application example of the above-described hierarchicalencoding system, when a high-vision TV still picture database isconstructed, data in the lowest hierarchical level, namely, data in thefirst hierarchical level (original picture), is reproduction data with ahigh-vision resolution. Data in the second hierarchical level isreproduction data with a standard resolution. Data in the highesthierarchical level, namely data in the fifth hierarchical level, isreproduction data with a low resolution for high speed data retrieval.

When a compression encoding process is used for reducing the informationamount, reproduction picture data obtained by the decoding apparatusdoes not always accord with the input original picture data. However,the deterioration of the picture quality can be suppressed so that itcannot be visually detected. In addition, the average value may beobtained by a simple averaging method or a weighted averaging method.

The present invention can be applied for the quantizing process of thedifference signal generated by other than the above-described predictionencoding process. In addition, the present invention can be applied fora system having a buffering construction for controlling a quantizingstep width Δ so as to control the generated data amount.

Having described specific preferred embodiments of the present inventionwith reference to the accompanying drawings, it is to be understood thatthe invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one skilledin the art without departing from the scope or the spirit of theinvention as defined in the appended claims.

According to the present invention, as the number of quantizing steps,an odd number can be used. In addition, according to the presentinvention, when difference data (difference signal) is quantized with 0as a decoded value, the quantizing characteristic has a symmetry withrespect to 0. Moreover, according to the present invention, the degreeof freedom of designing a quantizing apparatus is increased and therebythe encoding efficiency can be improved.

What is claimed is:
 1. Hierarchical image coding apparatus for codingimage data, comprising:a converting portion for converting a first imagedata to a second image data, a number of pixels of the second image databeing less than a number of pixels of the first image data; a predictingportion for predicting a predicted first image data from the secondimage data; a subtracting portion for obtaining difference data betweenthe first image data and the predicted first image data; and aquantizing portion for determining a number of quantizing steps for thedifference data and for obtaining a quantizing code by quantizing thedifference data in response to each of the determined quantizing stepsincluding odd-numbered steps, said quantizing portion including aconverting table for storing a respective variable-length quantizedvalue in correspondence with a respective number of quantizing steps anda respective quantizing code such that said quantizing portion producesa variable-length quantized value of the difference data based on thecorresponding number of quantizing steps and the correspondingquantizing code, the difference data being represented by acorresponding variable-length quantized value.
 2. The apparatusaccording to claim 1, further comprising:another quantizing portion forquantizing the second image data received from said converting portionto produce the quantized second image data; and a dequantizing portionfor dequantizing the quantized second image data to produce dequantizedsecond image data, wherein said predicted portion predicts the predictedfirst image data from the dequantized second image data.
 3. Theapparatus according to claim 1, wherein said converting portion producesthe second image data by averaging the first image data.
 4. Hierarchicalimage coding apparatus for coding image data, comprising:a convertingportion for converting a first image data to a second image data, anumber of pixels of the second image data being less than a number ofpixels of the first image data; a predicting portion for predicting apredicted first image data from the second image data; a subtractingportion for obtaining difference data between the first image data andthe predicted first image data; and a first quantizing portion forquantizing the difference data, said first quantizing portion includinga detecting portion for detecting a local characteristic of thedifference data and for selecting a plurality of quantizing stepsincluding odd-numbered steps based on the detected local characteristic;a second quantizing portion for obtaining a quantizing code byquantizing the difference data in response to each of the selectedquantizing steps, said second quantizing portion including a convertingtable for storing a respective variable-length quantized value incorrespondence with a respective number of quantizing steps and arespective quantizing code so as to produce a variable-length quantizedvalue of the difference data based on the corresponding number ofquantizing steps and the corresponding quantizing code; and anoutputting portion for outputting said variable-length quantized valuewith the corresponding quantizing step.
 5. The apparatus according toclaim 4, further comprising:another quantizing portion for quantizingthe second image data received from said converting portion to producethe quantized second image data; and a dequantizing portion fordequantizing the quantized second image data to produce dequantizedsecond image data, wherein said predicted portion predicts the predictedfirst image data from the dequantized second image data.
 6. Theapparatus according to claim 4, wherein said converting portion producesthe second image data by averaging the first image data.
 7. Hierarchicalimage coding method for coding image data, comprising the stepsof:converting a first image data to a second image data, a number ofpixels of the second image data being less than a number of pixels ofthe first image data; predicting a predicted first image data from thesecond image data; obtaining difference data between the first imagedata and the predicted first image data; determining a number ofquantizing steps for the difference data; and obtaining a quantizingcode by quantizing the difference data in response to each of thedetermined quantizing steps including odd-numbered steps using aconverting table, the converting table storing a respectivevariable-length quantized value in correspondence with a respectivenumber of quantizing steps and a respective quantizing code so as toproduce a variable-length quantized value of the difference data basedon the corresponding number of quantizing steps and the correspondingquantizing code, the difference data being represented by acorresponding variable-length quantized value.
 8. The method accordingto claim 7, further comprising the steps of:quantizing the second imagedata to produce the quantized second image data; and dequantizing thequantized second image data to produce dequantized second image data,wherein the predicted first image data is predicted from the dequantizedsecond image data.
 9. The method according to claim 7, wherein said stepof converting includes averaging the first image data to produce thesecond image data.
 10. Hierarchical image coding method for coding imagedata, comprising the steps of:converting a first image data to a secondimage data, a number of pixels of the second image data being less thana number of pixels of the first image data; predicting a predicted firstimage data from the second image data; obtaining difference data betweenthe first image data and the predicted first image data; detecting alocal characteristic of the difference data; selecting a plurality ofquantizing steps including odd-numbered steps based on the detectedlocal characteristic; obtaining a quantizing code by quantizing thedifference data in response to each of the selected quantizing stepsusing a converting table, said converting table storing a respectivevariable-length quantized value in correspondence with a respectivenumber of quantizing steps and a respective quantizing code so as toproduce a variable-length quantized value of the difference data basedon the corresponding number of quantizing steps and the correspondingquantizing code; and outputting said variable-length quantized valuewith the corresponding quantizing step.
 11. The method according toclaim 10, further comprising the steps of:quantizing the second imagedata to produce the quantized second image data; and dequantizing thequantized second image data to produce dequantized second image data,wherein the predicted first image data is predicted from the dequantizedsecond image data.
 12. The method according to claim 10, wherein saidstep of converting includes averaging the first image data to producethe second image data.